In order to avoid any ambiguity the term "optical fibre" is defined to include any light-conducting optical component in the form of a filament comprising a part forming a waveguide and referred to as the "core", at least partially surrounded by a sheath, it being possible for the core to extend coaxially with the sheath or to be offset or out of line, and it being possible for the sections of this core and of this sheath independently to be both circular and parallelepipedal, or of other shapes (for example in the form of a truncated circle, as in the patent U.S. Pat. No. 4,867,522).
In the text which follows, reference will nevertheless essentially be made, for the sake of clarity, to substantially cylindrical optical fibres having a core and a sheath which are both of circular cross-section, extending concentrically, in the axis of the fibre under consideration.
Progress optical fibres and in their applications has given rise to the development of components based on fibres fulfilling the same functions or at least functions which are analogous to those customarily fulfilled by conventional optical components.
Other fibre-optic components are more specific, and it is difficult to find their counterparts in conventional optics.
For this reason, optical fibres having a Bragg lattice have been developed.
Such fibres are described in the American patents U.S. Pat. Nos. 4,725,110, 4,806,012 and 4,807,950.
According to the technique described in these documents, a fibre, of glass or of silica doped with germanium, is treated in such a manner as to constitute within the fibre a Bragg grating which is permanent or definitively modified.
This lattice is composed of plane and parallel strata of constant index. The index varies from one stratum to the next, and its variations are periodic, of period I, the strata accordingly not being in this case ephemeral, and thus not disappearing when the fibre is no longer illuminated.
It is shown, in these documents, that the luminous flux reflected by such as Bragg lattice has characteristics which are a function of its period.
The period is determined by the conditions of manufacture of the Bragg lattice.
In order to be permanent, this Bragg lattice is constructed by subjecting an optical fibre to the interference lattice produced by the intersection of two ultraviolet beams originating, in the particular case concerned, from one and the same source, the high intensity of the light waves applied and their application conditions (wavelength, angles of propagation, etc.) permitting the creation of a fibre core exhibiting a refractive index which is definitively modified by impressing thereon or by etching thereon (at least) one Bragg lattice in strata, almost irreversible periodic perturbations of the refractive index then being formed in the material constituting the core of this fibre.
The various strata of the fibre are subjected to variable illuminations which involve variable modifications of index. As the distribution of the intensity within the interference fringes is sinusoidal, of period I, they produce variations of index of the same period within the fibre.
The period of the Bragg grating is accordingly determined by the conditions of production of the interference (angle, wavelength of the beams, etc.).
It is emphasized, in these documents of the prior art, that the fibres are especially intended to be utilized as a strain gauge.
The object of the present invention is the design of a novel optical component which is simple and capable of very widely varying applications.